Differential amplifiers are commonly employed in electronic devices that use analog circuits. In addition to a variety of discrete circuit applications, differential amplifiers are also used in many integrated devices such as, for example, operational amplifiers, which are a fundamental building block in many analog circuits and devices. The growing demand for mobile or portable electronic equipment or devices has increased the need to produce simple, lightweight, energy-efficient electronic equipment, which has resulted in an increased demand for low-power operational amplifiers.
Generally speaking, to reduce the power consumption of an operational amplifier, the operational amplifier must be operated at relatively low supply voltages. Unfortunately, as the supply voltage is reduced, the useful dynamic input range and output range of the operational amplifier is reduced. In general, the operating range of the input terminals of an operational amplifier depends on the input stage configuration of the operational amplifier. As is well known, the operating range or dynamic range of the input terminals of a differential amplifier is commonly referred to as a common-mode input range (CMR). In the case of an operational amplifier buffer circuit such as, for example, a voltage follower, the CMR of the operational amplifier determines the dynamic range of the buffer inputs. A differential amplifier that provides a CMR substantially equal to the voltage drop across the supply terminals of the differential amplifier is commonly referred to as a rail-to-rail CMR differential amplifier.
FIG. 1 shows a fully differential input stage 100 which is commonly used in an operational amplifier. NMOS devices MN1 and MN2 are ideally identical, as are the PMOS devices MP1 and MP2. When input signals IN and IP are at the same DC voltage, output signals ON and OP should have the same DC level which is a known common mode level. In reality, however, because of process variations, the DC potentials of the output signals ON and OP are not the same and not well-defined. Particularly, because the PMOS and NMOS devices have inherent differences due to the fabrication process, DC outputs cannot be well-defined in the circuit 100 of FIG. 1, even when the input signals IN and IP are at the same DC value. That is, the output signals OP and ON both could drift in the same direction to the stronger PMOS or NMOS device. This drift could degrade the gain of amplifier because the stronger devices tend to go into the unwanted bias region such as the low resistance triode region.
Common Mode Feed Back (CMFB) circuits are used to create a well-defined DC common mode output level. A typical CMFB circuit 200 is shown in FIG. 2. By sensing the DC output voltage of the output signals OP and ON and by comparing the output signals OP and ON with the desired output DC voltage level Vcm, the output signals OP and ON are adjusted to be the same as Vcm via the feedback loop. However, present CMFB circuits can only correct the common mode output voltage level. They cannot correct the output offset voltage.
Furthermore, because of process variations, NMOS devices MN1 and MN2 become un-identical, as do the current mirror MP1 and MP2. Mismatches between these same-type devices create unequal outputs. When input signals IP and IN are at the same DC value, output signals OP and ON are different. When the input signals IP and IN are equal, the DC voltage difference between the output signals OP and ON is known as the output offset voltage.
What is therefore needed is a CMFB circuit that also provides output offset reduction/cancellation.